The present invention relates to a resin composition for a sliding member and a sliding member produced from the resin composition, and more particularly, to a resin composition for a sliding member which exhibits excellent friction and wear characteristics even at a low sliding speed under a high load, and a sliding member produced from the resin composition.
Conventionally, there have been proposed various synthetic resin sliding members made of a thermoplastic synthetic resin blended with a lubricating oil, which are further enhanced in self-lubricating property and wear resistance and capable of withstanding a long-time use without applying oils thereto.
For example, the inventions of Japanese Patent Publication (KOKOKU) Nos. 46-5321(1971), 46-42217(1971) and 47-42615(1972) relate to a process for the production of a sliding member made of a polyacetal resin containing a lubricating oil, and those KOKOKUs describe that a polyacetal resin is impregnated with a lubricating oil in order to reduce its friction coefficient and remarkably enhance its wear resistance. However, these conventional processes have technical problems from the standpoint of production engineering, and further suffer from problems such as failure to lower its friction coefficient (static friction coefficient) upon initiation of sliding operation, deterioration in friction and wear characteristics at a sliding speed higher than 25 m/minute, rough surface of molded products with the increase in its lubricating oil content, or the like.
To solve the above conventional problems, the techniques described in Japanese Patent Application Laid-Open (KOKAI) No. 62-45662(1987) have been proposed. In this KOKAI, there is described a synthetic resin composition for a sliding member which is prepared by uniformly mixing a thermoplastic synthetic resin with 1 to 10% by weight of a saturated fatty acid, 1 to 20% by weight of a phosphate powder and not more than 2% by weight of a lubricating oil wherein the weight ratio of the lubricating oil to the saturated fatty acid is {fraction (1/10)} to xc2xd.
Although the siding member produced from the synthetic resin composition described in Japanese Patent Application Laid-Open (KOKAI) No. 62-45662(1987) can exhibit excellent friction and wear characteristics, it has been further found that in the case where the sliding member is applied to sliding portions such as steering device of automobiles, there arise such a problem that its friction coefficient becomes high and its wear amount becomes large under specific use conditions, for example, at a sliding speed as low as not more than 1 m/minute under a load (surface pressure) as high as more than 50 kgf/cm2. In addition, the sliding portions such as steering device of automobiles are inevitably exposed to vibrations transmitted from outside. Therefore, it has been required that the sliding member itself exhibits a good vibration-absorbing property.
As a result of the present inventor"" earnest studies for solving the above problems, it has been found that by blending a polyester-based elastomer exhibiting a good vibration-absorbing property by its rubber elasticity with specific amounts of fatty acid, metallic soap, phosphate and lubricating oil, a sliding member produced from the thus obtained resin composition can exhibit not only excellent friction and wear characteristics even at a low sliding speed under a high load, but also an excellent vibration-absorbing property. The present invention has been attained on the basis of the above finding.
An object of the present invention is to provide a resin composition for a sliding member capable of exhibiting excellent friction and wear characteristics even at a low sliding speed under a high load.
Another object of the present invention is to provide a sliding member which can exhibit excellent friction and wear characteristics even at a low sliding speed under a high load and an excellent vibration-absorbing property.
To accomplish the aims, in a first aspect of the present invention, there is provided a resin composition for a sliding member comprising from 0.1 to less than 1.0% by weight of a fatty acid, 0.1 to 2.0% by weight of a metallic soap, 0.1 to 2.0% by weight of a phosphate, 0.3 to 2.0% by weight of a lubricating oil, and a polyester-based elastomer as the balance.
In a second aspect of the present invention, there is provided a sliding member produced from the resin composition as defined in the first aspect.
The present invention will be described in detail below. First, the polyester-based elastomer as a main component of the resin composition for a sliding member according to the present invention will be explained.
As the polyester-based elastomer, there may be exemplified elastomers comprising polybutylene terephthalates (PBT) as hard segment, and aliphatic polyethers or aliphatic polyesters as soft segment.
The elastomers composed of polybutylene terephthalates (PBT) and the aliphatic polyethers are so-called polyester-polyether type elastomers. Examples of such polyester-polyether type elastomers may include xe2x80x9cHYTRELxe2x80x9d (tradename, produced by DuPont Co.), xe2x80x9cPELPRENE P-TYPExe2x80x9d (tradename, produced by Toyo Boseki Co., Ltd.), or the like.
Also, The elastomers composed of polybutylene terephthalates (PBT) and the aliphatic polyesters are so-called polyester-polyester type elastomers. Examples of such polyester-polyester type elastomers may include xe2x80x9cPELPRENE S-TYPExe2x80x9d (tradename, produced by Toyo Boseki Co., Ltd.), or the like.
The fatty acids usable in the present invention are higher fatty acids exhibiting a wax-like or oily state at ordinary temperature and having usually not less than 10 carbon atoms, preferably 10 to 34 carbon atoms. As such higher fatty acids, there may be used either saturated fatty acids having usually not less than 10 carbon atoms, preferably 10 to 34 carbon atoms, or unsaturated fatty acids having usually not less than 12 carbon atoms, preferably 12 to 24 carbon atoms.
As the saturated fatty acids, there may be used normal saturated fatty acids exhibiting a wax-like state at ordinary temperature and having usually not less than 10 carbon atoms. Examples of the normal saturated fatty acids may include capric acid (C10), lauric acid (C12), myristic acid (C14), palmitic acid (C16), stearic acid (C18), arachic acid (C20), behenic acid (C22), cerotic acid (C26), montanic acid (C28), melissic acid (C30) or the like.
As the unsaturated fatty acids, there may be used those exhibiting an oily state at ordinary temperature and having usually not less than 12 carbon atoms. Examples of the unsaturated fatty acids may include lauroleic acid (C12), myristoleic acid (C14), oleic acid (C18), linoleic acid (C18), gadoleic acid (C20) or the like.
The amount of the fatty acid blended may be determined according to amounts of the below-described other components blended. The fatty acid can exhibit an effect of reducing a friction coefficient of the obtained resin composition even when blended in as small an amount as 0.1% by weight. The lower limit of the amount of the fatty acid blended is usually 0.1% by weight, preferably 0.4% by weight, more preferably 0.5% by weight based on the weight of the resin composition. The upper limit of the amount of the fatty acid blended is usually less than 1% by weight, preferably 0.9% by weight, more preferably 0.8% by weight based on the weight of the resin composition. When the amount of the fatty acid blended is not less than 1% by weight, although the effect of reducing the friction coefficient can be exhibited, the obtained resin composition is deteriorated in wear resistance at a low sliding speed under a high load. When the amount of the fatty acid blended is less than 0.1% by weight, the effect of reducing the friction coefficient cannot be exhibited. These fatty acids may be blended and uniformly mixed in the form of powder, granules, butter-like paste or oil, with the above polyester-based elastomer. In the case of solid fatty acids, it is preferred to mix the solid fatty acid with the polyester-based elastomer while stirring and heating to such a temperature capable of melting the solid fatty acid.
The metallic soap used in the present invention may include at least one metallic soap selected from the group consisting of calcium soap, sodium soap, aluminum soap and lithium soap.
The metallic soap exhibits not only the effect of reducing the friction coefficient similarly to the above fatty acid, but also shows such an effect of absorbing or adsorbing and retaining the below-described lubricating oil and preventing the bleed-out of the lubricating oil upon molding. The amount of the metallic soap blended is usually 0.1 to 2.0% by weight, preferably 0.4 to 1.0% by weight based on the weight of the resin composition. When the amount of the metallic soap blended is less than 0.1% by weight, neither the effect of reducing the friction coefficient nor the effect of preventing the bleed-out of the lubricating oil can be exhibited. When the amount of the metallic soap blended is more than 2.0% by weight, the obtained resin composition is deteriorated in heat stability.
As the phosphate used in the present invention, there may be exemplified metal salts such as tertiary phosphates, secondary phosphates, pyrophosphates, phosphites and metaphosphates, or mixtures thereof. Among these phosphates, tertiary phosphates, secondary phosphates and pyrophosphates are preferred. As metals contained in the phosphates, there may be exemplified alkali metals, alkali earth metals and transition metals. Among these metals, alkali metals and alkali earth metals are preferred, and lithium (Li), calcium (Ca), magnesium (Mg) and barium (Ba) are more preferred.
Specific examples of the especially preferred phosphates may include trilithium phosphate (Li3PO4), dilithium hydrogenphosphate (Li2HPO4), lithium pyrophosphate (Li4P2O7), tricalcium phosphate (Ca3(PO4)2), calcium pyrophosphate (Ca2P2O7) and calcium hydrogenphosphate (CaHPO4(xc2x72H2O)).
The phosphate may be uniformly mixed with the polyester-based elastomer, the fatty acid and the metallic soap. The phosphate has an average particle size of usually not more than 20 xcexcm, preferably 3 to 10 xcexcm. Although the phosphate itself does not show a lubricating property, when the phosphate is used together with the fatty acid in a small amount and the below-described lubricating oil, there can be obtained such synergistic effects of stabilizing the friction coefficient and enhancing the wear resistance. The amount of the phosphate blended is usually 0.1 to 2.0% by weight, preferably 0.4 to 1.2% by weight based on the weight of the resin composition. When the amount of the phosphate blended is more than 2.0% by weight, the obtained resin composition is deteriorated in wear resistance. When the amount of the phosphate blended is less than 0.1% by weight, the effect of improving the wear resistance cannot be exhibited.
As the lubricating oil used in the present invention, there may be exemplified mineral oils such as machine oils and engine oils, vegetable oils such as castor oil and jojoba oil, synthetic oils such as ester oils and silicone oils, or the like. The lubricating oil can exhibit a synergistic effect of reducing the friction coefficient when used together with the above metallic soap. The amount of the lubricating oil blended is usually 0.3 to 2.0% by weight, preferably 0.4 to 1.6% by weight based on the weight of the resin composition. When the amount of the lubricating oil blended is less than 0.3% by weight, the effect of reducing the friction coefficient cannot be exhibited. When the amount of the lubricating oil blended is more than 2.0% by weight, the obtained resin composition is deteriorated in a moldability such as a biting property (a property whether the molding material can advance smoothly through a cylinder by rotating screw).
The resin composition prepared by blending specific amounts of the respective components together is injection-molded or extrusion-molded in order to produce a sliding member such as a bearing. The thus obtained sliding member has a smooth and glossy surface, and is free from flow marks and rough surface.
The sliding member of the present invention preferably has a kinetic friction coefficient of 0.14 to 0.18; a static friction coefficient of 0.10 to 0.13; and an wear amount of 5 to 10 xcexcm when subjected to the following thrust test. That is, in the thrust test, the end face of a cylindrical mating member made of carbon steels for machine structural use (S45C) is pressed against a square plate-shaped molded sliding member, and then the cylindrical mating member is slidingly rotated at a sliding speed of 0.8 m/minute under a load of 100 kgf/cm2 without lubrication for 8 hours.
The resin composition for a sliding member according to the present invention can exhibit excellent friction and wear characteristics even when used at a low sliding speed under a high load. Also, the sliding member produced from the resin composition has a smooth and glossy surface and is free from flow marks and rough surface.